You should be able to find one pretty easily when you get to Nashville. It's derby season. Just go to the nicest department store. If you don't have time to shop when you get there, maybe just try a large department store. Where I'm from, there's always a hat section, but I don't believe they're as popular up north.
I really think you are going to have to go to a department store and try some on to see what looks best and fits you (but I had a little fun looking anyway)